Discuss the diagonal relationship of lithium with magnesium.

(N/A) The similarity between lithium and magnesium is particularly striking and arises because of their similar ionic sizes and charge-to-size ratios (ionic potential).
Atomic radii: $Li$ $(152 \ pm)$; $Mg$ $(160 \ pm)$
Ionic radii: $Li^{+}$ $(76 \ pm)$; $Mg^{2+}$ $(72 \ pm)$
The main points of similarity are:
$(i)$ Both lithium and magnesium are harder and lighter than other elements in their respective groups.
$(ii)$ Lithium and magnesium react slowly with water. Their oxides and hydroxides are much less soluble, and their hydroxides decompose on heating. Both form a nitride, $Li_{3}N$ and $Mg_{3}N_{2}$, by direct combination with nitrogen.
$(iii)$ The oxides, $Li_{2}O$ and $MgO$, do not combine with excess oxygen to give any superoxide.
$(iv)$ The carbonates of lithium and magnesium decompose easily on heating to form the oxides and $CO_{2}$. Solid hydrogen carbonates are not formed by lithium and magnesium.
$(v)$ Both $LiCl$ and $MgCl_{2}$ are soluble in ethanol.
$(vi)$ Both $LiCl$ and $MgCl_{2}$ are deliquescent and crystallise from aqueous solution as hydrates, $LiCl \cdot 2H_{2}O$ and $MgCl_{2} \cdot 8H_{2}O$.